1. The Field of the Invention
This invention relates to devices and methods used to make dimensional measurements using video cameras. More particularly, the present invention relates to making dimensional measurements of a laser beam which are used to determine important parameters and characteristics of the laser beam.
2. The Background Art
A variety of video cameras accompanied by appropriate signal processing equipment are advantageously used to make quantitative dimensional measurements in scientific and industrial fields. For example, video cameras based upon charge coupled devices (CCD) and vidicon devices are commonly used to make dimensional measurements. Cameras based upon CCD and vidicon devices are rugged mechanisms with many desirable characteristics for use in making quantitative dimensional measurements.
Laser devices have become common in modern technology. A laser beam provides a source of highly concentrated optical energy which has found use in many diverse applications in research, medicine, and industry. The primary features that make a laser beam useful for such applications include, for example: a single wavelength or color, i.e., monochromatic light; collimation of the optical energy so that the laser beam travels in a narrow beam over a very long distance; the ability to concentrate large amounts of energy in a very short period of time; and, the ability to focus the laser beam to a very small spot.
The laser's capability to control the exact spot size and spatial distribution of laser beam energy is of critical importance in nearly all applications of lasers. In order to control the spot size and spatial distribution of laser beam energy it is necessary to measure the spatial energy profile, and in particular, the precise dimensions of the laser beam. It is only when accurate measurements of the spatial profile and dimensions of laser beams are possible that the performance and effectiveness of a laser beam can be significantly improved.
Video cameras using CCD (charge coupled devices), CID (charged injection devices), self-scanned arrays, vidicon tubes (video cameras using vidicon tubes sometimes being referred to as vidicon cameras) and other camera technologies have become widely used as part of a system to measure laser beam dimensions as well as to carry out other optical dimensional measurements. When using video cameras to measure laser beam parameters, the laser beam is directed to impinge upon the video camera's light sensitive surface and the video camera is able to record the spatial intensity profile of the laser beam over its entire two-dimensional matrix. The signal which is output from the video camera is then subjected to a video digitizer and appropriate digital signal processing hardware and software to provide laser beam parameter measurements.
The video cameras and associated signal processing equipment provide two principal functions when performing laser beam measurements. The first function is the ability to record and display the laser beam profile. Visualization or displaying of the laser beam profile assists a laser user by providing an intuitive feel for the results of quantitative measurements. The fast response of such video cameras and signal processing equipment can provide a display of laser beam profiles in real time and in both 2D and 3D modes.
The second function is to make quantitative measurements on laser beam parameters. Such laser beam parameters include total power, power density, and in particular, the width or diameter of the laser beam. These detailed quantitative measurements of laser beam characteristics allow a user to precisely determine the properties of the laser beam and to make incremental adjustments and improvements in its performance.
Video cameras using CCDs, hereinafter referred to as CCD cameras, in particular have become popular as a tool for conducting laser beam diagnostic measurements. The ability of the CCD camera to simultaneously measure the entire surface area of the laser beam and perform detailed spatial measurements makes it well suited for conducting laser beam diagnostics. These cameras are often used by scientists and engineers who are either designing lasers or who are using lasers in applications where the spatial profile of the laser beam is critical.
The before-mentioned video cameras, in spite of their advantages, possess certain characteristics that limit the precision with which laser beam diagnostics can be carried out. Still, some of the properties of CCD cameras make them well suited for spatial profile measurements of laser beams. These video cameras, however, have some characteristics that limit their usefulness in laser beam diagnostics and in other industrial quantitative measurement applications.
First, CCD cameras and vidicon cameras typically have a low signal-to-noise ratio, even when the signal is approaching saturation, which causes problems in obtaining precise measurements under varying camera conditions.
Second, some video cameras possess a measurement error resulting from a fixed baseline offset error inherent in the camera. The fixed baseline offset error exacerbates the seriousness of the low signal-to-noise ratio. It has been shown (Jones, R., Laser Focus World, January 1993) that a 1% error baseline offset error can contribute up to 20% error in the measured width of a laser beam. Moreover, fixed baseline offset can also come from signal processing equipment.
Third, some video cameras, and particularly CCD cameras and vidicon cameras, exhibit what is referred to as "shading error" in the baseline which results in the baseline offset changing from one portion of the spatial plane in the camera to another.
These inherent characteristics have limited the ability of some commercial grade video cameras to make accurate measurements. Generally, accurate measurements using video cameras is possible only when the signal produced by the camera is very close to saturation and only when the signal covers a relatively large area of the light sensitive surface in the video camera.
The problems encountered with video cameras such as CCD cameras and vidicon cameras are accentuated in laser beam diagnostics applications because the measured dimensions of the laser beam impinging upon the light sensitive surface of the camera is highly dependent upon the proper detection of low level beam intensities in the wings, or outer regions, of the laser beam where the signal-to-nose ratio of the video camera is inherently very low and may even be less than one. In contrast, the integrated total energy in the wings can be significant due to the relatively large area of the wings found in some laser beams. In cases where the wing portion of the laser beam is large or exhibits high energy the noise which is inherent in video cameras, especially in the presence of baseline offset error and shading errors, can create very large errors in calculated beam dimensions.
In addition to the above noted difficulties with video cameras, there are a number of different definitions for laser beam width which must be considered. Some of the definitions for laser beam width are useful for some types of laser beams, and other definitions are useful for other types of laser beams. Nevertheless, for a great majority of laser beams, many of the definitions have an equivalence to one another. Information on the equivalence of various definitions of laser beam width is available from Seagman, Johnston and Sassnet, "Choice of Clip Levels for Beam Width Measurements using Knife-Edge Technologies," IEEE Journal of Quantum Electronics (April 1991). The most useful of these definitions of laser beam width, and their accompanying methods, are known in the art and will be described below.
One fundamental approach to defining laser beam widths is based upon the second moments of the energy distribution in a two-dimensional intensity profile. This definition of laser beam width is referred to as the D.sub.4.sigma. approach. However, the calculation of the D.sub.4.sigma. beam widths is compromised due to the limitations of the recording devices, e.g., the videocameras and signal processing equipment. In particular, noise and baseline offset have a stronger effect on creating errors when using the second moment definition. Thus, other definitions of laser beam widths are used because they agree with the d.sub.40 definition to within a high degree of accuracy under certain conditions and are less vulnerable to the limitations of the recording devices.
Another definition of beam width is called D.sub.86 and is one in which all of the pixels which are measuring a signal in the beam are summed, starting at the highest magnitude, until 86% of the total energy impinging on the light sensitive surface of the video camera is counted. At this point, a diameter is calculated based upon the area containing those pixels which make up 86% of the total energy of the laser beam. Disadvantageously, the shape of the beam is assumed to be circular and only a single diameter is computed.
A third definition of laser beam width is called knife-edge. The knife-edge definition is one in which an equivalent knife-edge is drawn across the signal from the video camera until it cuts off 10% of the energy of the laser beam. The knife-edge continues to be drawn across the signal from the video camera until it cuts off 90% of the laser beam. The laser beam width is defined as the distance between 10% and 90% positions multiplied by a correction factor relating it to the second moment value. The knife-edge and D.sub.86 definitions function to approximate the second moment definition widths of a laser beam.
A fourth definition of laser beam width is referred to as aperture wherein an aperture is drawn around the laser beam centroid. The size of the aperture is increased until it includes 86% of the energy of the laser beam whereupon the size of the aperture is taken as the diameter of the laser beam.
A fifth definition of laser beam width is known as Full Width Half Max (FWHM) and is performed by making a measurement on the width of the laser beam at the positions that are exactly half the energy of the peak. Alternatively, the Full Width Half Max definition can be altered so that the percent energy of the peak is defined by the user. A common percent is 13.6%, which corresponds to the 1/e.sup.2 point, and on a perfect Gaussian beam provides results which are equivalent to those obtained using the D.sub.86 definition or the corrected knife-edge definition described above.
The commonly accepted beam widths for non-top hat laser beams fall into two basic categories, second moment and FWHM. The D86, knife-edge, and aperture definitions are all attempts at finding a the second moment beam width equivalent. In all of the above mentioned definitions, except FWHM and some alteration of FWHM, any baseline offset error or shading present in the video camera has a very significant effect on the resulting measurement. For example, if a relatively small laser beam covers 10.times.10 pixels out of a 120.times.120 pixel array, the total integrated energy in the beam, if the peak is saturated, would be a maximum of 25,600 digital counts using an 8-bit digitizer. If the baseline offset were in error by only 1 digitizer count out of 256, the baseline would contribute 14,400 counts, or more than half the signal in the laser beam. Thus, a beam width measurement that integrated energy in the area of the beam, could be in error by more than 100%. Thus, it is critical that the baseline of the camera be properly set in order to make valid laser beam width measurements.
In view of the foregoing, it would be an advance in the art to provide a method and apparatus which overcomes the noted problems and which provides improved dimensional measurements using commercially available video cameras.